The present application claims priority under 35 U.S.C. xc2xa7119 of German Patent Application No. 100 03 685.6, filed on Jan. 28, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety.
1. Field of the Invention
The invention relates to a former of a machine for producing a tissue web containing two circulating continuous dewatering belts that converge to form a stock inlet nip and which are guided subsequently over a forming element such as, e.g., a forming roll. The invention also relates to a process of making a tissue web using such a former.
2. Discussion of Background Information
Double wire formers and crescent formers are typically used for the production of tissues or tissue webs. In this way, a fibrous suspension is introduced between two dewatering belts which are guided over at least a part of the circumference of a forming roll. In the case of double wire formers, the two dewatering belts are embodied as dewatering wires. In the case of crescent formers, however, a felt belt is provided as the inner wire while outer belt is an outer wire.
Arranged behind the forming roll when viewed in the flow direction, the outer belt is guided away from the inner belt. The tissue web formed between the two dewatering belts separates from the upper dewatering belt and travels along with the lower dewatering belt. However, this separation process causes problems when the outer belt is very rough and/or very open. Such problems can even occur when the wire is one with a zonally variable wire permeability, i.e., a so-called DSP wire. Apparently, the dewatering flow partially pulls the fibers into the dewatering belt, where they interlock. This is disadvantageous since the tissue web may undesirably tend to travel along with the outer wire. Furthermore, it is also disadvantageous because the fibers, which are interlocked with the outer wire, are not completely removed together with the tissue web from the rough open outer wire during the separation process, but instead partially remain in the outer wire, i.e., undesirably stuck to the outer wire. During operation, this incomplete separation problem between the fibers and the outer wire leads to an increasing contamination of the outer wire, and thus to a loss of quality in the web being produced. Runability problems can also occur in the tissue machine as a result or incomplete separation.
The invention therefore provides a former as well as a process of the type mentioned at the outset that ensures a reliable and secure operation of a tissue machine containing a DSP wire in its initial dewatering section.
Accordingly, the invention provides for a former which utilizes a dewatering wire with zonally variable wire permeability that is provided as the outer belt. This outer wire does not come into contact with the forming element. Further, in the area of a separation point, wherein this outer belt and the inner belt are separated from one another, a suction element is provided inside the loop of the inner belt. The forming element may itself be the suction element by being provided with a suction zone and/or the suction element may be positioned downstream from the forming element.
By utilizing a suction element, the tissue web can be securely lifted off the outer wire. In this regard, a suction box with edges and suction slits, a suction roll, and/or a suction pipe, for example, may be used as a suction element.
Wires having zonally variable permeability are known, e.g., as disclosed in Swedish Patent SE 427 053 which is expressly incorporated by reference in its entirety. According to this document, such wires can be formed as, e.g., a woven material in which longitudinal and lateral threads are provided on one or more levels, i.e., woven together according to a predetermined pattern such that systematically distributed areas of suitable size develop in which the number of overlapping points equals zero or is significantly smaller than in the woven structure of the remaining woven material.
According to one embodiment of the former according to the invention, the vacuum provided in the suction element is adjustable. Thus, a problem-free adjustment of each operational condition (basis weight, porosity of the web, speed) is possible.
The suction element is suitably positioned, in the web travel direction, in front of the separation point.
The separation of the two webs can occur, for instance, in the area of and/or on the suction element.
The suction element may also be advantageously embodied such that it affects the inner belt at least essentially over its entire width.
The suction element may accordingly be provided with, for instance, suction apertures which are positioned such that the tissue web reaches the suction zone before the dewatering belts are separated. Further, the suction apertures may have the form of, for instance, one or more suction slits which are as wide as the machine or several rows of apertures consecutively positioned in the web travel direction and offset in relation to each other (e.g., round holes, oblong holes, etc).
In order to improve or support the effective separation of the suction elements, a blowing element may also be provided in the area of the separation point, e.g., positioned inside the loop of the outer belt. This blowing element may suitably be embodied such that it affects the outer belt at least essentially over its entire width. The blowing medium may be water, air, or the like; however air is a preferred medium.
According to another embodiment, the forming element is provided with a suction zone in which a corresponding vacuum is preferably adjustable. Using the adjustable vacuum allows for the contact intensity between the tissue web and the inner belt to be modified and/or adjusted. By utilizing an increased vacuum, for instance, the adhesion of the web to the outer belt can be reduced. Moreover, this acts to increase the adhesion between the tissue web and the inner belt, which improves the separation accordingly.
According to another aspect of the invention, the former includes at least one of the two dewatering belts which is embodied as a dewatering belt with zonally variable wire permeability, i.e., a DSP wire and a conditioning device which may be a wire cleaning device. The conditioning device being assigned to such a dewatering wire. This design counteracts the tendency for contamination of the DSP wire, which is based on fibers invading the volume of the wire and sticking due to its variable permeability. Moreover, this design reduces corresponding contamination of the wire, which affects the performance of the wire, and thus the quality of the tissue web. Thus, the disadvantages which occur because of contamination are avoided by the conditioning device and/or wire cleaning device according to the invention. According to one embodiment, the conditioning device may have the form of, for instance, spray pipes having jets provided over the width of the machine. A traversing conditioning device may also be used, e.g., a xe2x80x9cDuocleanerxe2x80x9d (cf. EP-O 731 212 A) made by the company Voith Sulzer and having both rotating high pressurized jets and integrated suctioning. A xe2x80x9cJet Cleanerxe2x80x9d made by the company Voith Sulzer, for instance, may also be used. U.S. Pat. No. 5,783,044 illustrates a xe2x80x9cDuocleanerxe2x80x9d which may be used; this document being expressly incorporated by reference in its entirety.
The conditioning device is preferably embodied such that it affects the dewatering belt at least essentially over its entire width. Moreover, a cleaning device operating accordingly over the entire width (e.g., spray pipes with jets positioned over the width of the machine, Jet Cleaner, cleaning roll, etc) can be used in combination with a traversing locally intense operating cleaning device (e.g., Duocleaner, high pressure jet). By combining such cleaning elements, wire wear can be minimized at the same time that sufficiently good cleaning effects are maintained.
According to another embodiment, the corresponding former is embodied as a crescent former in which the inner belt is provided as a felt belt.
According to one embodiment of the process according to the invention, a dewatering belt having zonally variable wire permeability is used as the outer belt, wherein this outer belt does not come into contact with the forming element. Additionally, the outer belt and the inner belt are separated from one another in the area of the separation point. Moreover, the inner belt is suctioned by one or more suction elements provided inside its loop; and/or the forming element is provided with a suction zone and acts as the suction element.
This design acts to counteract the tendency for contamination of the corresponding DSP wire.
Advantageous embodiments of the process according to the invention may be as follows: the inner belt may be suctioned in the web travel direction in front of the separation point; the inner belt may be suctioned at least essentially over its entire width; the outer belt may be affected by a blowing element positioned inside its loop in the area of the separation point; and the outer belt may be affected by a blowing element at least essentially over its entire width.
According to one embodiment, the process according to the invention is used advantageously in a crescent former.
Moreover, wires of the type described in WO 00/12817 (application No. PCT/GB99/02684) which is expressly incorporated by reference in its entirety, for example, can be used as wires having the zonally variable permeability. The correlating wires can particularly comprise a woven material in which threads provided in one or several levels and running in the one direction are woven with threads running in another direction, such that a mesh results that separates a multitude of systematically distributed areas of predeterminable configurations and accordingly determines with the systematically distributed areas each containing three threads running in the one direction and at least three running in the other. In particular, the threads can be woof threads and warp threads.
The invention therefore provides a former for producing a tissue web, comprising a forming element, an inner dewatering belt, and an outer dewatering belt, the inner and outer belts converging to form a stock inlet nip, the inner and outer belts being guided over the forming element and thereafter separating from one another in the area of a separation point, and at least one suction element positioned adjacent the inner belt on a side which is opposite the outer belt.
At least the outer belt may be a dewatering wire having zonally variable wire permeability. The tissue web may be separated from the outer belt in the area of the separation point. The tissue web may be retained by the inner wire after being separated from the outer belt. At least one of the inner and outer belts may comprise a circulating continuous dewatering belt. The forming element may comprise a forming roll. Each of the inner and outer belts may be a circulating continuous dewatering wire having zonally variable wire permeability. The inner belt may contact the forming element and the outer belt may be guided with the inner belt around the forming element such that the outer belt does not come into contact with the forming element. The forming element may comprise the at least one suction element. The forming element may comprises a suction zone. The at least one suction element may be positioned adjacent the area of the separation point. The at least one suction element may be provided inside a loop of the inner belt. The at least one suction element may comprise a vacuum suction element and wherein the vacuum present inside the suction element is adjustable. The at least one suction element is positioned in front of the separation point, in a web travel direction. The at least one suction element may cause the inner belt to separate from the outer belt. The at least one suction element may be arranged at least essentially over an entire width of one of the inner belt and the tissue web. The former may further comprise at least one blowing element positioned adjacent the outer belt on a side which is opposite the inner belt. The at least one blowing element may be located in the area of the separation point. The at least one blowing element may be located in the area of the separation point and inside a loop of the outer belt. The at least one blowing element may be arranged at least essentially over an entire width of one of the outer belt and the tissue web. The forming element may comprise a suction zone having adjustable vacuum.
The invention also provides for a former for producing a tissue web, comprising a forming element, an inner dewatering belt, and an outer dewatering belt, the inner and outer belts converging to form a stock inlet nip, the inner and outer belts being guided over the forming element and thereafter separating from one another in the area of a separation point, and a conditioning device positioned adjacent the outer belt.
At least one of the inner belt and the outer belt may be a dewatering wire having zonally variable wire permeability. The former may further comprise at least one suction element positioned adjacent the inner belt on a side which is opposite the outer belt. The tissue web may be separated from the outer belt in the area of the separation point. The tissue web may be retained by the inner wire after being separated from the outer belt. At least one of the inner and outer belts may comprise a circulating continuous dewatering belt. The forming element may comprise a forming roll. Each of the inner and outer belts may be a circulating continuous dewatering wire having zonally variable wire permeability. The inner belt may contact the forming element and the outer belt may be guided with the inner belt around the forming element such that the outer belt does not come into contact with the forming element. The forming element may comprise the at least one suction element. The forming element may comprise a suction zone. The former may further comprise at least one suction element positioned adjacent the area of the separation point. The at least one suction element may be provided inside a loop of the inner belt. The at least one suction element may comprise a vacuum suction element and wherein the vacuum present inside the suction element is adjustable. The at least one suction element may be positioned in front of the separation point, in a web travel direction. The at least one suction element may cause the inner belt to separate from the outer belt. The at least one suction element may be arranged at least essentially over an entire width of one of the inner belt and the tissue web. The former may further comprise at least one blowing element positioned adjacent the outer belt on a side which is opposite the inner belt. The at least one blowing element may be located in the area of the separation point. The at least one blowing element may be located in the area of the separation point and inside a loop of the outer belt. The at least one blowing element may be arranged at least essentially over an entire width of one of the outer belt and the tissue web. The forming element may comprise a suction zone having adjustable vacuum. The conditioning device may comprise a wire cleaning device. The conditioning device may be arranged at least essentially over an entire width of one of the outer belt and the tissue web. The inner belt may be a felt belt. The former may be a crescent former.
The invention also contemplates a process for producing a tissue web in a former which includes a forming element, an inner dewatering belt, and an outer dewatering belt, the inner and outer belts converging to form a stock inlet nip, the inner and outer belts being guided over the forming element and thereafter separating from one another in the area of a separation point, and at least one suction element positioned adjacent the inner belt on a side which is opposite the outer belt, the process comprising forming the tissue web in the area of the forming element, guiding the inner and outer belts around the forming element, and separating the inner and outer belts in the area of the separation point.
The separating may comprise separating the outer belt from the inner belt. The separating may comprise suctioning the inner belt via the at least one suction element which is positioned inside a loop. The separating may comprise suctioning the inner belt via the forming element, the forming element comprising the at least one suction element having a suction zone. The separating may comprise suctioning the inner belt in front of the separation point, in the web travel direction. The separating may comprise suctioning the inner belt via the at least one suction element arranged at least essentially over an entire width of one of the tissue web and the inner belt. The process may further comprise blowing a medium against the outer belt using a blowing element positioned adjacent the outer belt in the area of the separation point. The blowing may comprise blowing a medium against the outer belt via the at least one blowing element arranged at least essentially over an entire width of one of the tissue web and the outer belt. The blowing element may be positioned inside a loop of the outer belt in the area of the separation point.
There is also provided a process for producing a tissue web in a former which includes a forming element, an inner dewatering belt, and an outer dewatering belt, the inner and outer belts converging to form a stock inlet nip, the inner and outer belts being guided over the forming element and thereafter separating from one another in the area of a separation point, and a conditioning device positioned adjacent the outer belt, the process comprising forming the tissue web in the area of the forming element, guiding the inner and outer belts around the forming element, and conditioning the outer belt.
The process may further comprise separating the inner and outer belts in the area of the separation point. The separating may comprise separating the outer belt from the inner belt. The separating may comprise suctioning the inner belt via the at least one suction element which is positioned inside a loop. The separating may comprise suctioning the inner belt via the forming element, the forming element comprising the at least one suction element having a suction zone. The separating may comprise suctioning the inner belt in front of the separation point, in the web travel direction. The separating may comprise suctioning the inner belt via the at least one suction element arranged at least essentially over an entire width of one of the tissue web and the inner belt. The process may further comprise blowing a medium against the outer belt using a blowing element positioned adjacent the outer belt in the area of the separation point. The blowing may comprise blowing a medium against the outer belt via the at least one blowing element arranged at least essentially over an entire width of one of the tissue web and the outer belt. The blowing element may be positioned inside a loop of the outer belt in the area of the separation point. The former may comprise a crescent former in which the inner belt is a felt belt. The at least the outer belt may be a dewatering wire having zonally variable wire permeability.
The invention farther provides for a former for producing a tissue web, comprising a forming roll, an inner continuous dewatering belt, and an outer continuous dewatering belt, the inner and outer belts converging to form a stock inlet nip, a headbox positioned adjacent the stock inlet nip, each of the inner and outer belts forming corresponding inner and outer continuous loops which are each guided over a plurality of guide rolls, each of the inner and outer belts being guided over the forming roll and thereafter separating from one another in the area of a separation point, and one of at least one suction element positioned inside the inner loop and adjacent the inner belt on a side which is opposite the outer belt and a conditioning device positioned adjacent so as to clean the outer belt, wherein at least one of the inner and the outer belts is a dewatering wire having zonally variable wire permeability.
The former may further comprise a press nip through which the tissue web and the inner belt is guided, the press nip being formed between a cylinder and shoe press roll, wherein the tissue web is removed from the inner belt after passing through the press nip.
Other exemplary embodiments and advantages of the present invention may be ascertained by reviewing the present disclosure and the accompanying drawing.